1. Field of the Invention
The present invention relates generally to devices and methods for forming scanned electronic images, more specifically, to swath scanners and a programmed method for reconciling captured data to ameliorate banding at swath boundaries.
2. Description of Related Art
Scanners for creating an electronic image of an original print or document are well known in the state of the art. Typically, a captured image provided by scanning an original with a drum, flatbed, two-dimensional array, or hand-held scanner is in the form of picture element ("pixel") data, comprising a data matrix, or array, that is stored in a memory in a digital format. In other words, with a scanner, an original image is captured and converted into a digital light-intensity map for computer processing. In this two-dimensional map of pixels, each pixel holds an intensity measurement corresponding to the optical information, in reflectance or transmittance values, of the scanned object at the physical location represented by the pixel. The scanner captures each pixel with as many as 36-bits to describe the gray levels or color gradations of the original. In the state of the art for both scanning and printing the captured data--such as with an ink-jet printer--a pixel can be as small as 1/600-inch across.
Data acquisition and processing to produce a faithful reproduction of the original presents many problems to scanner and printer system engineers. In conversion from a typical color scanner, producing 3-bytes (24-bits) of light intensity level data for each pixel (1 of 2.sup.8 =256 possible levels), to ink-jet printing where only 3-bits are necessary to describe the presence or absence of a color dot of cyan, magenta, and yellow primary color ink or black ink, the total of eight printer dot colors to more than 16-million (2.sup.24) possible scanner-pixel colors requires substantial data manipulation to print an acceptable rendering.
In swath scanning where the original is scanned swath-by-swath (x-axis) orthogonally to the direction of the paper movement (y-axis), problems are added to the faithful reproduction aspect in that swath interface regions, or boundaries, are subject to error. In other words, while a page-size, two-dimensional scanner has the luxury "seeing" whole rows of the image pixels at once and therefore having gradual, relatively invisible, changes in illumination levels for the data acquisition task, the moving scanner has the added problem of aligning and putting together, or algorithmically "stitching," sets of swaths. This can result in a clear and unacceptable errors in a scanned rendition. For example, in FIG. 1A--a copy of a Kodak.TM. test photograph pattern--distinct swath interface regions, or "swath boundary discontinuities," generally referred to in the art as "banding," can be seen with the naked eye (e.g., the vertical bands are particularly noticeable in the caption line which in the original test print has a continuous hue), presenting a good, yet flawed, copy of the original.
Photometer signal measurements--measuring the incident light and converting the measured value to an analog voltage used by an analog-to-digital converter to produce each pixel data point--are susceptible to even small changes in distance between the photometer and the illuminated source since intensity falls proportional to the distance squared. For example, near paper pinch rollers, relatively large deformations of the transported media occurs, making the distance between the document being scanned and the sensors different at the edges of the document than at the middle where it is generally held flat against a platen. Any calibration error incurred from the start of the scan of a swath, pixel location 1 hereinafter, to the end of the swath, pixel location M, will result in noticeable visual discontinuities in the final rendering.
Mechanical fixes to this problem generally relate to trying to hold the paper profile constant between the calibration pass (for example, the setting of the sensitivity limits of a white level (highest intensity, normalized to a "1") and black level (lowest intensity, normalized to a "0")) and the actual scan passes. Alternatively, devices try to provide an illumination which is insensitive to changes in paper height. While generally inexpensive, and therefore commercially desirable fixes, the eye's sensitivity to the problem is so keen that removal of all visible banding due to this phenomena by mechanical means only have not been totally successful. Furthermore, the problem can be exacerbated in a curved paper path device which has a dual function, such as an ink-jet printer with an interchangeable scanner head, where a curved platen is preferable for printing processes but undesirable for scanning processes. Additionally, paper profiles will vary depending on the thickness of the media, making a calibration for one medium unsuited for use with the other.
For color image scanning, another solution is provided by the use of multi-pass scanning where three exposures (red, green, blue) are captured in each pass and the data recombined using color comparison correction algorithms. While one-pass/one-exposure scanners offer added flexibility and speed but less critical image reproduction, multiple-pass/multiple-exposure scanners offer better image quality but at the expense of throughput. Examples of various multi-pass image rendering techniques are found in U.S. Patent Nos. 5,644,683, 5,512,923 5,140,432 4,999,646, 4,967,203 (each assigned to the common assignee of the present invention).
Another problem caused by swath boundary discontinuities is that it makes it difficult to apply image processing to the scan data, particularly for sharpening an image.
Thus, there is a continuing need for improvement in the data acquisition and processing techniques for swath scanners in order to improve scan quality. There is a need for a system less sensitive to paper height changes during scanning.
The term "programmed" as used herein for description or claims is intended to include any of various electronic methods of performing the recited functions or logical steps, such as with the use of software, firmware, state machines, application specific integrated circuits ("ASICs"), and the like as would be familiar to a person skilled in the art of computer and computer peripherals technology.